1. Field of the Invention
The present invention relates to bond magnets used as high performance permanent magnets and manufacturing methods thereof, and actuators therewith such as motors.
2. Description of the Related Art
So far, as one kind of high performance permanent magnets rare earth system magnets such as Sm--Co system magnets and Nd--Fe--B system magnets are well known. A large amount of Fe and Co in these magnets contributes to increase saturation magnetic flux density. The rare earth elements such as Nd and Sm, due to a behavior of 4f electrons in a crystal field, cause very large magnetic anisotropy. Thereby, coercive force is increased.
The rare earth magnets of high performance are employed in electrical appliances mainly such as motors and measuring instruments. Recently, in order to cope with demands for smaller sizes and lower prices of various kinds of electrical appliances, permanent magnets of higher performance are in demand. In particular, in medium driving spindle motors used in hard disk drives (HDDs), floppy disk drives (FDDs), CD-ROM drives and DVD drives and actuators for driving light pick-ups used in CD-ROM drives and DVD-drives, in realizing smaller size and higher performance, further improved permanent magnet is in demand.
To the demands for permanent magnets such as described above, for instance R--Zr--(Fe, Co)--N system magnet material (R: rare earth elements) of which principal phase is TbCu.sub.7 type crystal structure is proposed (cf. Japanese Patent Laid-Open Application (KOKAI) No. HEI 6-172936 and others). A permanent magnet using such magnet material is in general a bond magnet constituted of for instance a molded body (compression molded body) that is obtained by compression-molding a mixture of magnet powder and resin-based binder into a molded body. The mixture of the magnet powder and the resin-based binder is press-formed by use of a press machine of general use.
Magnetic performance of the bond magnet such as described above, residual magnetization and maximum magnetic energy product ((BH).sub.max) in particular, is determined by magnetic properties of the magnet material being used and density of the molded body (compression molded body) constituting the bond magnet. That is, when the magnet material of equivalent performance being used, by increasing the density of the molded body, the bond magnet can be made higher performance. When the density of the molded body is increased to decrease voids, corrosion-resistance of the bond magnet is also expected to improve.
However, in a manufacturing method of the bond magnet therein an existing general press forming is applied, there is a limit in an improvement of the density of the compression molded body consisting of the mixture of the magnet powder and the binder. In particular, as in the magnet material of which principal phase is TbCu.sub.7 type crystal phase, when flakes (or a ribbon) of magnet material obtained by quenching are used, during press forming, there occurs friction or spring back between magnet materials. Thereby, the compression-molded body is hindered from being densified. Accordingly, the bond magnet is in a situation difficult to further improve performance due to an improvement of the density of the molded body.